Viscosimeters are generally known, for example, the Ostwald viscosimeter (Glasstone, Textbook of Physical Chemistry, 2nd Ed., 10th Printing (1954) D. vam Nostrand Co., page 498), comprising a capillary adapted to the flow of the liquid to be determined. Such a viscosimeter is adapted only to measurements on liquids having a constant viscosity and it cannot be used for measurements of liquids having a varying viscosity, e.g. due to a reaction in the liquid, such as an enzymatically decomposing starch or protein solution having a decreasing viscosity, a polymerizing solution having an increasing viscosity, or a DNA solution in which the viscosity varies due to influences of temperature, pH or ion concentration.
The generally known method of the falling ball (Glasstone, loc. cit. page 498 and 499) is not suitable for this purpose either.
It is an object of the invention to provide a low feed, fast response, very low time of delay viscosimeter which, in particular, is suitable for continuous measurements of liquids showing a viscosity varying with the time.
Thus, the invention provides a viscosimeter comprising a flow system for the liquid to be tested, having a narrow zone, zones for determining the pressure of the liquid to be tested located before and after the said narrow zone, said zones for determining the pressure of the liquid to be tested being surrounded by tubes of a thin-walled material adapted to transfer the pressure of the liquid within the tubes to pressure transduction liquids outside each of the tubes, each of said pressure transduction liquids being in contact with one of two pressure chambers of a pressure difference recording means in such a manner that the pressures of the liquid to be tested at both sides of the narrow zone are transferred to said pressure chambers.
The tubes of thin-walled material are within chambers containing the pressure transduction liquid, said chambers being connected to connecting tubes, each of them leading one of the two pressure chambers of the pressure difference recording means.
The viscosimeter according to the invention enables in an easy manner to follow continuously the viscosity of a liquid flowing through the narrow zone, preferably a capillary, at constant speed.
The relationship between the viscosity and the pressure gradient over a capillary may be derived from the Hagen-Poiseuille equation for a liquid flowing through a capillary: 8n ##EQU1## in which .eta. = viscosity of the liquid
R = radius of the cross-section of the capillary PA1 V = volume of the liquid flowing through the capillary within a period t PA1 P.sub.1 - p.sub.2 = pressure difference of the liquids at both sides of the capillary PA1 L = length of the capillary
From this equation, which is a good approximation when a laminary flow is maintained in the capillary, it may easily be concluded that, at constant flow speed, the viscosity is proportional to the pressure difference, and that variations in viscosity thus are indicated by the variations of the pressure difference which may be transferred, for example, into an electrical signal by means of a device acting according to the inductive, capacitive or piezoelectrical principle, which signal may be made visible by a recorder.
An advantage of the application of an additional liquid located between the spaces with the zones surrounded by the thin-walled, pressure-transferring material and the pressure difference meter, is that the liquid to be determined cannot remain in dead spaces and influence the measurement wrongly in this manner.
When, for example, quick changes occur in the viscosity of the liquid to be determined, the liquid present in the connecting tubes and diffusing into the liquid to be tested, would give rise to false measurements, and this may be avoided by the device of the invention.
Since the viscosity is generally highly dependent on the temperature, the capillary of the viscosimeter according to the invention is preferably provided with a jacket through which thermostatted liquid may be passed. It is also possible to place the whole viscosimeter in a thermostatted zone. Further, the liquid to be tested is preferably heated to the desired temperature by means of a thermostat before it is carried through the viscosimeter according to the invention.
The above-mentioned thin-walled, pressure-transferring material is preferably a flexible thin-walled rubber which is very useful for viscosity measurements in aqueous media. Another suitable material is, for example, polyvinylalcohol, enabling measurements is several organic solvents.
According to a preferred embodiment, the ends of the capillary are funnel-shaped and the zones surrounded by the tubes of thin-walled material are located in line with the capillary. Thus, the capillary may easily be cleaned by, e.g. mechanical means, without the necessity of demounting the apparatus.
It is appreciated that the viscosimeter according to the invention may also be used for measurements of the absolute viscosity; in that case, the viscosimeter has to be calibrated by means of two or more liquids having a known viscosity.